The present invention relates to sewage sludge disposal and, more specifically, to novel methods of dewatering and forming useful products from sludge produced by typical industrial or municipal sewage treatment systems.
Disposal of sewage sludges presents problems both of environmental pollution and of energy consumption. Although transporting sludges by barge several miles off coast and dumping it in the ocean is practised in many coastal areas, the resulting pollution is recognized as a serious environmental problem which cannot continue indefinitely without disastrous consequences. Likewise, land disposal by spreading wet sludges over unoccupied areas creates pathogenic and bacteriological risks, as well as esthetic problems. In any case, transportation and handling of the sludges, which may contain 90% or more of water, is expensive and energy consuming. In spite of the economic and ecological objections to water and land disposal of the sludges, they are considered preferable to drying or burning the sludge by heat from burning fuels due to the enormous quantity of energy required to evaporate the entrapped water.
Recent methods of treating sludge to reduce the disposal problems include acid treatment systems such as that disclosed in U.S. Pat. No. 3,772,191, wherein the sludge is acidified with a mineral acid and allowed to separate by gravity into an upper layer of hydrophobic substances, an intermediate layer of supernatant liquid, and a lower layer of sludge substances of lower water content than the original sludge. The upper and intermediate layers are drained off and the precipitated sludge substances, still containing 80% water, are treated with calcium hydroxide to facilitate further dehydration through pressure filtration in a filter press. The product mixture removed from the filter press has a water content of 30% to 50%, which is still 50-65% water based on contained sludge in the total mixture.
Although the above described process constitutes a material improvement over conventional disposal methods through improved pressure filtration rates of sludge solids, there are still a number of obvious problems. The surface layer, consisting of lipides, fatty acids, higher hydrocarbons and the like, must be disposed of, as must the intermediate layer of diluted acid. Also, the final product, consisting of sludge solids and lime, still may contain up to 50% water. Since the acidified sludge must be allowed to settle for several hours or days, the process is not suitable for continuous operation, with new materials constantly being added to mixing or reacting vessels.
It is a principal object of the present invention to provide a method for dewatering sewage sludge in a rapid, efficient, economical manner, essentially obviating all environmental and energy problems commonly associated with sludge disposal.
A further object is to provide a method of treating sewage sludge to obtain a dry end product having useful properties as a soil fertilizer, with no intermediate or by-products which create pollution or other environmental problems in their disposal.
Other objects are to provide a sludge treatment process which may be carried out with the apparatus of conventional fertilizer granulation plants without substantial modification; to provide a method of dewatering sewage sludge which may be practised in the manner of a continuous chemical process; to provide a method of dewatering sewage sludge which utilizes materials other than the sludge which presently constitute atmospheric pollutants; and, to provide a process for treatment of sewage sludge to yield a dry mixture of sludge solids and fertilizer salts wherein heat provided by a chemical reaction serves to sterilize the product.
Other objects will in part be obvious and will in part appear hereinafter.